Integrated circuits (ICs) are incorporated in many types of electrical equipment. Such ICs are generally vulnerable to damage from high voltage transients.
A source of high voltage electrostatic discharge (ESD) transients can result from a user becoming electrostatically charged, for example, by friction or by induction and touching the controls of the equipment.
Consequently, high voltage transients may be coupled to the terminals of ICs utilized in electrical equipment. Thus, IC's may be damaged by high voltage transients conducted by way of terminals.
Protection devices and circuitry applicable to the protection of ICs from damage that would otherwise result from a high voltage transient are known in the art. Such devices are described in, for example, the following patents, the disclosure of which is herein incorporated by reference: U.S. Pat. No. 4,066,918, (Heuner et al.), issued Jan. 3, 1978; U.S. Pat. No. 4,736,271, (Mack et al.), issued Apr. 5, 1988; U.S. Pat. No. 4,757,363, (Bohm et al.), issued Jul. 12, 1988; U.S. Pat. No. 4,896,243, (Chatterjee et al.), issued Jan. 23, 1990; U.S. Pat. No. 4,903,095, (Chapron), issued Feb. 20, 1990; and U.S. Pat. No. 4,922,371, (Gray et al.), issued May 1, 1990. Protection devices and circuits are also described in, for example, "High Density Input Circuit Design In 1.2 .mu.m CMOS Technology", Yean-Shan Hu et al., 1987 EOS/ESD SYMPOSIUM PROCEEDINGS, pages 179-185; and "Thick Oxide Device ESD Performance Under Process Variations", R. A. McPhee et al., 1986 EOS/ESD SYMPOSIUM PROCEEDINGS, PAGES 173-181.
It is herein recognized that in providing its protective function, it is desirable that the voltage at which a protection device exhibits a high level of conduction be sufficiently low, e.g. 15 volts, in order to provide significant protection of the device to be protected. On the other hand, a protection device should not exhibit a significant level of conduction below a voltage that is sufficiently high, e.g. 8 volts, in order to permit normal functioning of the device to be protected at normal operating voltages. It is also desirable that the protection device be able to handle transients associated with relatively large amounts of energy without itself being destroyed or having its protective capability significantly impaired. Furthermore, the protection device should exhibit fast turn-on and its structure should be compatible with the manufacturing process of the IC on which it is to be incorporated and preferably, it should not require any additional processing steps for its formation.